Japan reportedly lost track of a key secret North Korean negotiator several years ago, who had been directly connected to the leadership in Pyongyang. 

Citing sources familiar with the matter, Japan's Kyodo News reported on Thursday that the secret negotiator was known as "Mr. Y," and was a subordinate of "Mr. X," a high-ranking North Korean security official. 

Japan and North Korea have reportedly maintained a behind-the-scenes negotiation channel through Mr. X since around the 2002 visit to the North by then Japanese Prime Minister Junichiro Koizumi. 

Together with Mr. X, Mr. Y was reportedly involved in negotiations related to Koizumi's visit and also in charge of talks over a 2014 bilateral agreement struck in Stockholm on the settlement of the issue of Japanese abductees in the North. 

Mr. X was reportedly purged in 2011 and Mr. Y succeeded him. 

Kyodo News said that Mr. Y had told his contacts in Tokyo that he had fallen ill and the behind-the-scenes negotiation channel between the two nations has remained lost since. 

It was reported that he was believed to have developed a disease related to the brain or circulatory system, but could have been sacked from his post amid stalled Japan-North Korea negotiations.